The present invention relates generally to timing systems to measure short time intervals, and more particularly to timing systems suitable for time of flight pulse measurements such as found in systems used to guard protected equipment.
In many applications it is necessary to know the distance between two points. Although knowledge of distance per se can be used to make a range finder, in other applications knowledge of distance can be used to protect a zone against intrusion. A factory may have robotic or potentially hazardous equipment that is to be protected from outsiders. A system that can measure the distance between such equipment and a perimeter region around the equipment can sound an alarm or turn-off the equipment if anyone approaches closer than the periphery of the protected zone. In this fashion, outsiders are protected against harm from the equipment, and any operators using the equipment are protected from harm by being startled or otherwise disturbed by outsiders.
FIG. 1 depicts a generic so-called time-of-flight system 10 used to calculate the distance X between system 10 and a target (TARGET). System 10 may be located adjacent robotic or perhaps hazardous machinery in a factory where an alarm is to be sounded or the machinery turned-off if anyone approaches closer than distance X.
Typically system 10 includes a trigger generator 20 that creates a pulse train that is input to a transmitter (XMTR) 30, such as a high speed laser, that broadcasts a pulse via a suitable lens 40. The broadcast pulse 50 radiates outward at the speed of light, and at least a portion of the radiation may contact the surface of the target, and be reflected back toward system 10. The reflected-back radiation 55, which also travels at the speed of light, is detected by an appropriate transducer 60 (e.g., an optical lens) and photodetector 70. In a zone protection application, a mirror within system 10 mechanically rotates in a plane such that transmitted pulses scan the protected region, and return pulses are detected from this region. The protected region may be defined as a swept arc centered on the equipment to be protected, and extending outward with a radius of at least X. Typically the laser transmitter is triggered or pulsed with a known frequency in synchronism with mirror rotation such that detected return pulses can be correlated with an angle of emission, to locate the angular position and range of the intruding object. In such applications, any target (TARGET) within range X within the swept protection zone is presumed to be an intruder. Note that X may be a function of scan angle in that the guarded perimeter need not be defined by a swept arc.
As indicated in FIG. 1, there will be a phase or time shift between corresponding portions of the radiating pulse energy 50 and the return or reflected back radiation 55. Thus, at time t0 a first radiated pulse transitions 0-to-1, but the same pulse upon detection (denoted now P1xe2x80x2) will have its leading edge transition 0-to-1 at time t1+Tw later than t0. A high speed counter logic unit 80 within system 10 then attempts to calculate the difference in time between t1+Tw and t0. Tw is a signal strength dependent term that is sometimes called xe2x80x9ctiming walkxe2x80x9d.
Within unit 10, detected return pulse Pxe2x80x2 is amplified and coupled to a comparator to determine the return pulse transition timing. Return pulse transition timing is typically dependent on the strength of the return pulse, which in turn is determined by object reflectivity and range. In FIG. 1, T1 is the delay corresponding to the physical separation between system 10 and the object or target, whereas Tw is the timing walk strength dependent term.
Typically unit 80 includes a high speed master clock 85 (CLK) and a high speed counter 90 (COUNT). At time t0, as determined by a START pulse associated with the beginning of an output emission 50, counter 90 begins to count clock pulses. At time t1+tw, when pulse P1xe2x80x2 is detected, counter 90 is halted upon receipt of a STOP pulse, and the count value is determined.
Typically Tw is strongly dependent upon the signal response of the transmitter and receiver circuitry and must be characterized. Correction values are determined over a range of P1xe2x80x2 signal strengths and are stored in a table. The values stored in the correction table are indexed by detected signal strength and may be used by a system control circuit to extract the value t1. Thus, prior art systems that employ time-interval counters typically will use a peak-detector or signal integrator.
Once t1 is known, a measure of distance x given At xcex94t=(t1xe2x88x92t0) is determined by the following equation:   x  =                    c        ·        Δ            ⁢              xe2x80x83            ⁢      t        2  
where c=velocity of light (300,000 km/sec).
Within system 10, generating, transmitting, and receiving pulses can be straightforward. But it can be challenging for system 10 to resolve the distance X within a desired measurement granularity or tolerance. For example, to measure distance with a resolution granularity of about xc2x15 cm requires a 3 GHz counter. Such high speed devices are expensive and typically consume several watts of electrical power.
An alternative approach would be to replace the function of high speed clock 85 and high speed counter unit 90 with a high speed analog-to-digital converter. However high speed analog-to-digital converters are relatively expensive.
Yet another approach would be to replace units 85 and 90 with a transient recorder, perhaps inexpensively implemented using common CMOS fabrication processes. Transient recording could be extremely fast yet would not consume excessive electrical power. One prior art transient recorder technique is described in a Univ. of Calif. At Berkeley 1992 M. Sci. thesis entitled xe2x80x9cA Multi-Gigahertz Analog Transient Recorder Integrated Circuitxe2x80x9d by S. A. Kleinfelder. Kleinfelder""s thesis described a tapped, active delay line using an array of storage capacitors. The capacitors stored samples of the detected return pulse P1xe2x80x2 at specific delay times that were set by the delay of each element in the delay line.
Keinfelder""s approach appears ideal in that it presents a fully digitized representation of the delayed pulse (or multiple pulses), at relatively minimal cost. Further, no thresholding of the analog return pulse is necessary, and range distance may be computed using an algorithm that takes into account the full pulse shape. The latter is important in determining target range, independently of the strength of the return pulse P1.
Unfortunately, in practice Kleinfelder""s system is difficult to implement because of the large amount of data that must be processed in a relatively short time. Further, it is necessary to characterize performance of the active delay line and particularly the storage capacitors and analog-to-digital converter circuitry over process, temperature, and voltage variations.
What is needed is a high speed time interval measurement system for use in applications such as time-of-flight systems, especially in systems used to guard machinery or the like. Such measurement system should be inexpensive to fabricate, preferably using existing CMOS processes, should exhibit low power consumption, and should provide timing and strength information for one or more return pulses. Such measurement system should rapidly detect multiple return pulses, preferably within time intervals of less than about 500 ns, with a sub-nanosecond timing resolution that can provide spatial resolution of xc2x15 cm or less. Further, the system should measure return pulse signal strength with sufficient precision for use as an index to a lookup table to correct for timing walk. The system should communicate range measurements with a minimal amount of data. Finally, the system should exhibit reduced sensitivity to variations in ambient temperature, operating voltage, and fabrication processes.
The present invention provides such a high speed measurement system.
The present invention provides a high speed time interval system to measure time intervals xcex94t in time-of-flight measurement systems, preferably for use in systems that guard the perimeter of machinery or the like. The system measures time interval between a transmitted scanned laser pulse and a return pulse to determine distance, and can be fabricated on an integrated circuit (IC) using generic components. But unlike prior art systems, the system also returns a measurement of the width of the detected return pulse. Such information is used as indices to a lookup table that stores time walk corrections to the measured range distance. Knowledge of the return pulse width permits inferring strength of the return pulse, which inferred strength is used to estimate time walk Tw. Time walk Tw represents systematic error arising in raw range measurement due to fluctuations in detected signal strength and timing uniformities, the latter arising from process-dependent effects. The IC comprising the present invention is realizable with relatively inexpensive CMOS fabrication processes such that multiple data inputs may be incorporated into the IC without incurring significant additional cost.
The present invention is used with a zone protection system that includes a laser transmitter and photo detector that together define a coaxial field of view. A motor and mirror assembly cause emitted laser pulses and the detector field of view to scan a two-dimensional protection zone. The laser is pulsed with a specified frequency and in synchronously with motor-mirror rotation. A reference target is disposed within the zone protection system housing to reflect a portion of the transmitted energy back to the photo detector for use in compensating the present invention against system thermal drift.
In making time measurements, the present invention eliminates high speed clocks and high speed digital counters such as are commonly employed in prior art time-of-flight measurements. Instead, the present invention derives a START pulse from the laser drive signal (LASER START). This pulse is input to a latch whose output signal is propagated through a tapped delay line portion of a tapped delay line register (TDLR). The tapped delay line comprises preferably 512 buffers that each contribute an incremental time delay to the latch signal.
The detected return pulse is coupled to a bank of comparators (e.g., four comparators denoted CH0, CH1, CH2, CH3) that test the pulse against different threshold magnitudes. The comparator bank output signals are input to the TDLR, namely to individual channel registers that are also coupled to the preferably 512 delays from the tapped delay line register. The TDLR also receives the detector signal returned from the reference target. The TDLR is coupled to a microprocessor for readout, the microprocessor having access to a look-up table (LUT) that contains timing walk information that can be used to correct system errors.
The latched START pulse functions as a clock signal for the TDLR and clocks the detection data output from the comparator bank. The START-clocked TDLR, which preferably has four data inputs, functions in a manner similar to a two-bit transient recorder that clocks synchronously with the LASER START signal and can provide four signal values. For each data channel, the TDLR outputs a first over threshold (FOT) signal proportional to the rising edge of the first detected return pulse. FOT may be treated as the first 0-to-1 transition of the detected pulse signal. The FOT enables the microprocessor to provide a measure of time delay and thus of distance X to the target that returned the detected pulse. By itself the FOT can thus serve to approximate distance X. However the TDLR further determines and outputs a total time over threshold signal (TOT) that is proportional to how long the detected pulse energy exceeded a threshold. The TOT may be considered as how long the detected signal, after transitioning 0-to-1 remains at 1 before transitioning from 1-to-0. The TOT information permits the microprocessor to infer strength of the returned pulse, including rising and fall transition slopes and pulse width. The inferred strength information is used to index previously stored timing correction information in the look-up table. Such timing correction information will have been obtained during the system design by examining timing walk behavior of the system. The look-up table permits correction, as needed, to the FOT information. Further, the elimination of high speed clocks and high speed counters permits the system to be battery operated and fabricated as a single integrated circuit.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail, in conjunction with the accompanying drawings.